Earth's inner core, a red-hot ball of iron the size of Pluto, has stopped spinning in the same direction as the rest of the planet, and may even be spinning in a different direction. This is stated in a study published in Nature Geoscience.

 About 5,000 kilometers below the surface, this "planet within a planet" can spin independently as it floats in an outer core of liquid metal.

 What little is known about the inner core comes from measuring tiny differences in the seismic waves produced by earthquakes as they pass through the center of the Earth.

 In an effort to track the movements of the inner core, a new study published in the journal Nature Geoscience analyzed seismic waves from recurring earthquakes over the past six decades.

 The authors of the study, Xiaodong Song and Yi Yang of China's Peking University, said they found that the rotation of the inner core "almost stopped around 2009, and then turned in the opposite direction."

 "We believe that the inner core rotates with respect to the Earth's surface back and forth like a see-saw," they told AFP.

"One oscillation cycle is about seven decades," meaning it changes direction every 35 years, they added.

 They noted that it had previously reversed direction in the early 1970s and predicted that the next reversal would occur in the mid-2040s.

 The researchers said this rotation roughly coincides with changes in the so-called "length of day" -- small changes in the exact amount of time it takes Earth to rotate on its axis.

 The researchers said that, in their opinion, there are physical connections between all layers of the Earth, from the inner core to the surface.

 Experts who were not involved in the study expressed caution about its findings, pointing to several other theories and warning that many mysteries remain about the center of the Earth.

 "This is a very thorough study by excellent scientists, with a lot of data," said John Withale, a seismologist at the University of Southern California.

 "But none of the models, in my opinion, explain all the data very well," he added.

 Last year, Wiedale published research suggesting that the inner core oscillates much more rapidly, changing direction every six years or so. His work was based on seismic waves from two nuclear explosions in the late 1960s and early 1970s.

 Another theory, which Vidale says has strong evidence to support, is that the inner core moved significantly only between 2001 and 2013 and has remained in place since then.

 Hrvoje Tkalčič, a geophysicist at the Australian National University, published a study suggesting that the inner core cycles every 20 to 30 years, rather than the 70 years suggested in the latest study.